Combinatorial library screening and selection methods have become a common tool for identifying substrates or inhibitors of enzymes. The most widespread technique is phage display, whereby the protein of interest is expressed as a polypeptide fusion to a bacteriophage coat protein and subsequently screened by binding to immobilized or soluble biotinylated ligand. Phage display has been successfully applied to antibodies, DNA binding proteins, protease inhibitors, short peptides, and enzymes (Choo & Klug, 1995, Hoogenboom, 1997, Ladner, 1995, Lowman et al., 1991, Markland et al., 1996, Matthews & Wells, 1993, Wang et al., 1996).
Nevertheless, phage display possesses several shortcomings. For example, the nature of phage display precludes quantitative and direct discrimination of ligand binding parameters, such as quantitative characterization of protease specificity and substrate cleavage kinetics. Furthermore, some eukaryotic secreted proteins and cell surface proteins require post-translational modifications such as glycosylation or extensive disulfide isomerization which are unavailable in bacterial cells.
Accordingly, there remains a need in this art for systems and methods that provide efficient display and screening of polypeptides at the cell surface, as well as qualitative and quantitative characterization of the candidate polypeptides. The present invention addresses this need.